The present invention relates to a distortion canceling circuit for use in an amplifier; and, more particularly, to a distortion canceling circuit for reducing an unbalance between an upper 3rd-order distortion and a lower 3rd-order distortion generated by the amplifier.
In general, a distortion is generated in the course of amplifying a signal in an amplifier. Therefore, in a communications device which amplifies a communications signal by using an amplifier, there is needed to cancel a distortion caused in the course of amplifying a signal, e.g., a W-CDMA (wide-banded code division multiple access) signal or a multi-carrier signal, by using the amplifier.
FIG. 11 illustrates, as an amplifying device using a distortion canceling method of the prior art, an example of an amplifying device which cancels a distortion by using a distortion canceling circuit adopting a feed-forward method.
In the amplifying device shown in FIG. 11, an input signal (a main signal) is distributed in two ways by a distributor 81. A distributed signal is amplified by an amplifier (a main amplifier) 82 to thereby be inputted into a subtractor 84, and another distributed signal is inputted into the substractor 84 through a delay line 83. In the subtractor 84, a distortion component is extracted by deducting the signal inputted from the delay line 83 from the amplified signal inputted from the main amplifier 82. The distortion component is inputted therefrom into a distortion amplifier 85 and the amplified signal, which includes the distortion component, inputted from the main amplifier 82 is outputted to another subtractor 87 through another delay line 86. Further, the distortion component, which is extracted by the subtractor 84, is amplified by the distortion amplifier 85 to thereby be outputted to the subtractor 87. The subtractor 87 generates a final amplified signal without a distortion by deducting the amplified distortion component inputted from the distortion amplifier 85 from the amplified signal, which includes the distortion component, inputted from the delay line 86.
Here, the signal inputted from the delay line 86 to the subtractor 87 is generated by amplifying the input signal in the main amplifier 82 and the signal includes a distortion caused by the main amplifier 82. Further, the signal inputted from the distortion amplifier 85 to the subtractor 87 is generated by amplifying the distortion. Therefore, the output signal from the subtractor 87 is considered to be a signal generated by canceling the distortion caused by the main amplifier 82, i.e., deducting the distortion from the amplified signal generated by the main amplifier 82. Further, each of the distributor 81 and the subtractors 84 and 87 comprises, e.g., a directional coupler.
However, in such an amplifying device as described above, there is a problem of poor efficiency in the main amplifier 82. This is caused by a fact that an amplified signal outputted from the main amplifier 82 is attenuated while passing through the subtractor 84, the delay line 86 or subtractor 87, which requires an increase of an output level of the main amplifier 82 in accordance with a required output level of the amplifying device.
Meanwhile, FIG. 12 exhibits an example of an amplifying device having a distortion canceling circuit adopting a pre-distortion method.
The amplifying device of FIG. 12 has a pre-distortion circuit 91 coupled to an input of a main amplifier 92. The pre-distortion circuit 91 generates a distortion before a main signal being generated, a phase of the distortion having a difference of 180xc2x0 with respect to a distortion (i.e., an opposite phase) and a same amplitude as that of the distortion, the distortion being generated by the main amplifier 92. Therefore, the distortion caused by the main amplifier 92 is canceled by the distortion generated by the pre-distortion circuit 91.
Such an amplifying device can be implemented to obtain a high efficiency since any other circuits are not coupled to an output of the main amplifier 92. However, in this case, the distortion generated by the pre-distortion circuit 91 should have same characteristic as that generated by the main amplifier 92 regardless of the variation or frequency characteristics of an input signal.
Here, it is understood by those skilled in the art that the distortion caused by the main amplifier is due to an AMxe2x80x94AM (amplitude modulationxe2x80x94amplitude modulation) conversion or an AM-PM (amplitude modulationxe2x80x94phase modulation) conversion.
FIG. 13A charts a graph showing an example of the AMxe2x80x94AM conversion performed in the main amplifier. The horizontal and vertical axes of the graph represent an input level and a gain of the main amplifier, respectively. FIG. 13A shows an ideal gain characteristic (G1) together with those (G2 and G3) of the main amplifier and the pre-distortion circuit. As shown in FIG. 13A, the ideal gain characteristic (G1) can be obtained by combining those (G2 and G3) of the main amplifier and the pre-distortion circuit.
Further, FIG. 13B exhibits a graph showing an example of the AM-PM conversion performed in the main amplifier. The horizontal and vertical axes of the graph represent an input level and an output phase of the main amplifier, respectively. FIG. 13B shows an ideal phase characteristic (P1) together with those (P2 and P3) of the main amplifier and the pre-distortion circuit. As shown in FIG. 13B, the ideal gain characteristic (P1) can be obtained by combining those (P2 and P3) of the main amplifier and the pre-distortion circuit.
However, as shown in FIGS. 13A and 13B, characteristics of the AMxe2x80x94AM and the AM-PM conversion are so complicated that characteristic of the pre-distortion circuit must have a complicated function to implement an amplifying device having the above described ideal characteristics. Therefore, it is so difficult to calculate coefficients of the characteristic functions by using an analog approach.
Accordingly, as an alternative amplifying device having a distortion canceling circuit adopting the pre-distortion method, there has been proposed an amplifying device as shown in FIG. 14.
In the amplifying device of FIG. 14, an input signal, e.g., an RF (radio frequency) signal, is branched by a branch circuit 101. A branch signal is outputted from the branch circuit 101 to an amplitude/phase circuit 107 through a delay circuit 102. Another branch signal is outputted from the branch circuit 101 to an amplitude detector (envelope detector) 103.
The amplitude detector 103 detects an amplitude level (envelope level) of the inputted branch signal. And then, the detected amplitude level is converted into a digital signal by an A/D (analog to digital) converter 104. The digital signal is inputted to a table 105.
The table 105 stores data for the correction of an amplitude and a phase of a signal together with a corresponding amplitude level of the signal. Therefore, when the digitized amplitude level outputted from the A/D converter 104 is inputted to the table 105, corresponding data for amplitude and phase correction are read from the table 105 to thereby be outputted to a D/A (digital to analog) converter 106. The D/A converter 106 converts the data for amplitude and phase correction into an analog signal, the analog signal being inputted to the amplitude/phase circuit 107.
The branch signal, which is delivered from the branch circuit 101 to the amplitude/phase circuit 107 through the delay circuit 102, is synchronized with the data for amplitude and phase correction from the D/A converter 106.
Accordingly, in the amplitude/phase circuit 107, the delayed branch signal inputted from the delay circuit 102 is distorted in its amplitude and phase by using the data for amplitude and phase correction from the D/A converter 106. The amplitude and phase distortion imposed on the delayed branch signal by the amplitude/phase circuit 107 is subsequently cancelled by amplitude and phase distortion generated by a main amplifier 108. That is, the amplifying device has such a characteristic that AMxe2x80x94AM or AM-PM conversion is performed therein according to an input level thereof. However, as shown in FIGS. 13A and 13B, such a characteristic is cancelled by an opposite characteristic which is generated by the amplitude and phase correction data stored in the table 105, which results in an ideal characteristic of the amplifying device.
The signal amplified by the main amplifier 108 is outputted as a final output signal through another branch circuit 109. Further, in the branch circuit, a part of the amplified signal from the main amplifier 108 is branched to a distortion detector 110.
The distortion detector 110 extracts a distortion component remaining in the branch signal from the branch circuit 109 after the distortion canceling, the remaining distortion component being outputted to a table update circuit 111.
The table update circuit 111, based on the distortion component detected by the distortion detector 110, calculates amplitude and phase correction data for further canceling the distortion component remaining in the branch signal from the branch circuit 109. Subsequently, the amplitude and phase correction data are stored into the table 105. In this way, the amplitude and phase correction data stored in the table 105 are updated to minimize the amplitude and phase distortion caused by the amplifying device. Further, through the update of the amplitude and phase correction data by using the above described feed-back system, the amplifying device can operate in an efficient manner regardless of any effect caused by, e.g., a temperature change or a secular change.
FIG. 15 illustrates an example of amplitude and phase correction data, the values of which are optimized to cancel the distortion generated by the main amplifier, outputted from the table 105. The horizontal axis represents an envelope level of an input signal, i.e., an RF signal, (=an output level of the delay circuit 102). Higher and lower parts of the vertical axis represent an output value of the table 105 and time, respectively. That is, in FIG. 15, the horizontal axis and the lower part of the vertical axis make up a graph that shows a relationship between the time and the envelope level of the RF signal. Further, the horizontal axis and the higher part of the vertical axis make up a graph that exhibits a relationship between the output value of the table 105 and the envelope level of the RF signal. As shown in FIG. 15, when the envelope level of the RF signal changes with respect to the time, a value corresponding to the envelope level is outputted from the table 105.
Meanwhile, the degree of the distortion generated by the main amplifier depends on the frequency of the RF signal, which results in unpredictable characteristics of the distortion.
FIG. 16 shows an example of two output signals and corresponding distortions generated by a main amplifier when two input signals, each of which has a different frequency, e.g., f1 or f2, are inputted to the main amplifier. In FIG. 16, the horizontal and vertical axes represent frequency and amplitude level of the signals, respectively. FIG. 16 charts IM (intermodulation) distortion components, i.e., a lower 3rd order distortion and a higher 3rd order distortion at frequencies of (2xc2x7f1xe2x88x92f2) and (2xc2x7f2xe2x88x92f1), respectively, wherein f2 is larger than f1 (f2 greater than f1).
As shown in FIG. 16, when each of the amplitude levels of the two input signals has an identical value, there is introduced an amplitude difference xcex94IM (=Bxe2x88x92A) between an amplitude level (A) of the lower 3rd order distortion and that (B) of the higher 3rd order distortion at the frequencies of (2xc2x7f1xe2x88x92f2) and (2xc2x7f2xe2x88x92f1), respectively. In this case, although the pre-distortion circuit of the amplifying device operates in an ideal state, an identical distortion canceling process is performed with respect to the whole range of frequencies, such that a distortion component corresponding to the amplitude difference xcex94IM cannot be canceled by such a distortion canceling process.
Further, such an amplitude difference, xcex94IM, is caused by a distortion factor other than what the main amplifier has in general. For example, 3rd order distortion components, which are generated by the main amplifier, have the same amplitude levels at the frequencies of (2xc2x7f1xe2x88x92f2) and (2xc2x7f2xe2x88x92f1), respectively.
Even when ordinary distortion components, i.e., 3rd order distortion components, can be cancelled by the pre-distortion circuit having characteristics opposite to those of the 3rd order distortion components, xcex94IM shown in FIG. 16 cannot be cancelled by the pre-distortion circuit. For example, when A, B and xcex94IM are set to be 1.0, 0.8 and 2 dB=0.2, respectively, a distortion component other than the ordinary distortion component becomes 0.1 and the ordinary distortion component becomes {B+(Axe2x88x92B)/2}=0.9. In this case, since the distortion component other than the ordinary distortion component remains after distortion canceling performed by the pre-distortion circuit, the amount of distortion canceling becomes only |20 Log(0.1/0.9)|=19 dB. Further, the larger xcex94IM becomes, the smaller the amount of distortion canceling becomes.
Meanwhile, the amplifying device shown in FIG. 11, which has the distortion canceling circuit adopting the feed-forward method, can obtain an amount of distortion canceling of more than 30 dB. Therefore, as for the amount of distortion canceling, the amplifying device using the feed-forward method operates in a more efficient manner than that using the pre-distortion circuit.
There are several factors that cause the amplitude difference xcex94IM. For example, a distortion having a difference frequency of (f2xe2x88x92f1) is generated due to an even order distortion caused by a transistor included in the main amplifier, and the input signals having frequencies of f1 and f2 are modulated due to the distortion caused by the transistor. This is one of factors that may cause the amplitude difference xcex94IM. Such a distortion becomes more apparent when, as in an AB-class amplifier, the variation of drain current thereof is relatively large. Further, when an input signal having a frequency component of f1 or f2 is mixed with a signal having a frequency twice the frequency component, i.e., 2xc2x7f1 or 2xc2x7f2, the amplitude difference xcex94IM may be also introduced.
As described above, the amplifying device of the prior art, having the pre-distortion circuit for distortion canceling, has such a problem that there is an unbalance between the higher 3rd order distortion and the lower 3rd order distortion, wherein the unbalance cannot be canceled by the pre-distortion circuit.
It is, therefore, an object of the present invention to provide a distortion canceling circuit for canceling an unbalance between a higher 3rd order distortion and a lower 3rd order distortion generated when an input signal having more than two frequency components is amplified by an amplifier.
In accordance with a preferred embodiment of the present invention, there is provided a distortion canceling circuit for reducing an unbalance between a higher 3rd order distortion and a lower 3rd order distortion generated by an amplifier which amplifies an input signal having at least two frequency components, the distortion canceling circuit comprising: phase modulation means for performing phase modulation on the input signal by using a control signal having a frequency corresponding to a difference between the frequency components wherein the distortion canceling circuit cancels the unbalance by using side-band signals generated as a result of the phase modulation performed by the phase modulation means.
Here, the amplifier for amplifying the input signal having more than two frequency components is so called a common amplifier.
Further, as the phase modulation means, a phase modulator or a gate-source capacitance, which will be described later, can be used.
Further, the input signal may have, e.g., only two or more than three frequency components, or a frequency band which includes continuous frequency components.
Further, the unbalance between the higher 3rd order distortion and the lower 3rd order distortion means a phase difference or an amplitude difference therebetween.
Further, the difference between the frequency components of the input signal has a frequency component of (f2xe2x88x92f1) when the input signal has frequency components of f1 and f2. However, when the input signal includes more than three frequency components or a frequency band, the difference component is determined by summing differences between pairs of all the frequency components included therein.
In general, if 3rd order distortions can be expressed by using a two-frequency model, it is understood that it can be explained by using a multiple-frequency model. When n is a plural number, difference components of a signal having n frequency components can be detected by performing square detection. For instance, a signal having three frequency components, i.e., xcfx891, xcfx892 and xcfx893, can be expressed by performing square detection as follows: {cos(xcfx891xc2x7t)+cos(xcfx892xc2x7t)+cos(xcfx893xc2x7t)}2={cos(xcfx891xc2x7t)+cos(xcfx892xc2x7t)}2+2xc2x7cos(xcfx893xc2x7t)xc2x7{cos(xcfx891xc2x7t)+cos(xcfx892xc2x7t)}+{cos(xcfx893xc2x7t)}2. In the above equation, a difference component of (xcfx892xe2x88x92xcfx891) can be obtained from the first term of the right side. Further, difference components of (xcfx893xe2x88x92xcfx891) and (xcfx893xe2x88x92xcfx892) can be obtained from the second term of the right side.
In case higher 3rd order distortions and lower 3rd order distortions generated due to only a part of frequency components of the input signal are to be canceled, the difference components of the input signal, i.e., the control signal of the present invention, becomes differences between the part of frequency components. The present invention can be also applied for such a case.
Further, the control signal of the present invention is preferably generated from the input signal before being amplified by the amplifier. However, the control signal may be generated from the amplified input signal. That is, when the control signal is generated from the input signal, the control signal can closely follow the variation of the input signal. Meanwhile, although the control signal generated from the amplified input signal cannot follow the input signal as closely as the control signal generated from the input signal, the control signal can be also used for reducing the unbalance between the higher 3rd order distortion and the lower 3rd order distortion generated by the amplifier.
Further, in case the frequency components, i.e., f1 and f2, of the input signal can be determined prior to the input signal being inputted to the amplifier, a signal having a difference frequency, e.g., (f2xe2x88x92f1), can be also used as a control signal for the amplifier.
In the present invention, the side-band signals include a higher side-band signal and a lower side-band signal generated at frequencies of the higher 3rd order distortion and the lower 3rd order distortion, respectively. The present invention is able to reduce the unbalance between the higher and the lower 3rd order distortion by combining such a side-band signal with the higher and the lower 3rd order distortion generated by the amplifier.
Meanwhile, the phase modulation means of the present invention comprises a gate-source capacitance included in a transistor, the gate-source capacitance being varied according to a voltage of the control signal applied to a gate of the transistor. When the amplifier comprises a plurality of transistors, for example, at least one of the transistors can be the above-described transistor.
Preferably, the present invention cancels the higher 3rd order distortion and the lower 3rd order distortion generated in the amplifier by jointly using a pre-distortion circuit positioned in front of the amplifier and the phase modulation means.
On the other hand, firstly, the distortions generated by the amplifier are canceled by using the pre-distortion circuit, and then the higher and the lower 3rd order distortion each of which remains after the cancellation performed by the pre-distortion circuit is canceled by the phase modulation means.
Here, while the phase modulation means generates a higher side-band signal and a lower side-band signal between which there exists an unbalance, the pre-distortion circuit generates a higher 3rd order distortion and a lower 3rd order distortion each of which has the same amplitude and phase as those generated by the amplifier (i.e., there exists a balance therebetween). Although an ideal case is described herein, a pre-distortion circuit used in the state of the art does not necessarily produce balanced distortions. Such a characteristics of the pre-distortion circuit may be applied for the other descriptions thereon in the specification.
The pre-distortion circuit, for example, can include a combination of a variable attenuator for performing distortion canceling for the AMxe2x80x94AM conversion and a variable phase controller for performing distortion canceling for the AM-PM conversion.
Although it is preferable that the pre-distortion circuit positioned in front of the amplifier of the present invention is used together with the phase modulation means, a distortion canceling circuit positioned after the amplifier can be also used.
In general, a large portion of distortions caused by an amplifier having a series of transistors is generated in a transistor positioned at the end of the series. This is because the transistor positioned at the end of the series operates in a state of high efficiency, which causes a high output level of the transistor. However, when a front end of the series needs to operate in a state of high efficiency, a distortion canceling circuit can be inserted somewhere in between a first and a last transistor of the series. In a configuration where a distortion canceling circuit (post-distortion circuit) is positioned after the last transistor of the series, it is also necessary to increase the output level of the last transistor to compensate a loss of the distortion canceling circuit.
Further, although the present invention uses the side-band signals, which are generated due to the phase modulation of the input signal, to reduce the unbalance between the higher 3rd order distortion and the lower 3rd order distortion, the side-band signals can be also used for other purposes. Further, e.g., when the amount of the distortions is relatively small, the distortions can be canceled by using only the phase modulation.